Stabilization of multiple shaker systems

ABSTRACT

The problem of multiple shaker system instability due to crosscoupling of shaker drives via the load is significantly reduced by furnishing a bias signal which is a function of force application to the load via at least one shaker, and by applying the bias signal to bias the vibratory drive applied to the load via another shaker. The amplitude and phase of the bias signal are also controllable to effect reduction of instability.

"rs-e54 mte State 1A 1151 3,654,804 elmuth [4 Apr. 11, [54]STABILIZATION 0F MULTIPLE [56] References Cited SHAKER SYSTEMS UNITEDSTATES PATENTS [721 Inventor: James "elmuih, Mmlmvia, 2,844,777 7/1958Ross ..3 18/127 Ch d k-H l th El t I M 2,853,667 9/1958 Booth et al...3l8/l27 [73] Ass'gnee e cc 2,950,619 8/1960 Schuerch ..73/67.22,964,272 l2/l960 Olson ..248/19 [22] Filed: Apr. 8, 1970 3,015,9481/1962 Kearns ..73/67.2 [21] App! 26602 Primary ExaminerRichard C.Queisser Assistant ExaminerJohn P. Beauchamp Related Apphcauon DataAttorney-White, Haefliger & Bachand [63] Continuation-in-part of Ser.No. 606,657, Jan. 3,

1967, abandoned. ABSTRACT The problem of multiple shaker systeminstability due to [52] U.S.Cl ..73/7l.6,3l8/37, 318/128 cross couplingof Shaker drives via the load is significantly [51 1 III. Cl. reducedfurnishing a bias ignal which is a function of force [58] Field ofSearch ..73/7l.5, 71.6, 67.2, 67; application to the load via at leastone shaker, and by applying the bias signal to bias the vibratory driveapplied to the load via another shaker. The amplitude and phase of thebias signal are also controllable to effect reduction of instability.

Claims, 9 Drawing Figures L36 2g 3g me ma i- 4 c'mi ld SUMMINGPATENTEDAPR 11 me 3. 654,804

SHEEI 3 OF 3 .FIIG-S.

44 IA/wswroz (hue-.5 G. HELMU m i/M, wiw/ STABILIZATION OF MULTIPLESHAKER SYSTEMS This application is a continuation in part of my earlierapplication, Ser. No. 606,657, filed Jan. 3, 1967, now abandoned.

This invention relates generally to multiple shaker control systems, andmore particularly concerns stabilization of such systems.

In vibration testing of test specimens it is often-times desired to usemore than one shaker to excite a single specimen, as for example wherethe objective is to achieve a desired vibration level. As an example,certain advantageous multiple vibration exciter control systems aredisclosed in my co-pending application for U.S. Letters Patent entitled,MULTIPLE SHAKER CONTROL SYSTEM, Ser. No. 506,330, filed Nov. 4, 1965,now U.S. Pat. No. 3,477,281.

In multiple shaker systems of the type with which the present inventionis concerned,pcontrol of the phase and amplitude of the input to eachshaker is accomplished by detecting the in-phase and quadraturecomponents of the vibration pickup associated with each shaker drivepoint. The in-phase component is adjusted automatically by the amplitudecontrol system, or ACS; thus, the ACS increases the shaker drive inputto the load if the in-phase component is too small and vice versa. Inaddition, a phase control system (PSC) utilizes the quadrature componentdetected by the pickup to cause the drive phase angle to become moreleading (advances) if the pickup phase angle is lagging (retarded) andvice versa. However, in certain cases, asfor example resonant or tightlycoupled structures, instability and failure can occur when the vibrationpickup at a station is influenced to substantial extent by the drive tothe load from exciter or shaker heads other than the head associatedwith that pickup.

It is a major object of the present invention to provide method andapparatus for overcoming the instability problem defined above.Basically, the method of the invention involves furnishing a particularbias or stabilizing signal which is a function of theforce supplied tothe load via at least one of the shakers, and applying that signal tobias the vibratory drive applied to the load via another of the shakers,the amplitude and phase of the bias signal being such as to effectreduction of system instability. In one example, for the case where theshakers are substantially uniform and the transfer functions from theexciters to their corresponding control points are nearly identical, thebias signal may be furnished by deriving a summed version of multipledrive signals transmitted from multiple drives to multiple shakers, andinverting the phase of that summed version. By such summing, it becomespossible to use and adjust one gain control to achieve stability at anyfrequency.

As another example, for the case of a load structure and test set-up sonon-uniform that the above referred to transfer functions aresubstantially different, the bias or stabilizing signal may be derivedby recording the in-phase and quadrature components of the drive signalsto one shaker as a function of frequency. Thereafter, the recordedsignal is played back to bias the drive applied to the load via theother shaker, or other shakers. Such recordings may for example take theform of tape recordings, as will be described.

It is a still further object to provide circuit apparatus for performingthe stabilizing functions referred to above.

These and other objects and advantages of the invention, as well as thedetails of illustrative embodiments, will be more fully understood fromthe following detailed description of the drawings, in which:

FIG. 1 is a circuit diagram showing one preferred embodiment of theinvention;

FIG. 2a 2d are vector diagrams;

' FIG. 3 is a circuit diagram showing another embodiment of theinvention; and

FIGS. 4-6 are circuit diagrams showing still other forms of theinvention.

Referring first to FIG. 1, multiple shakers or exciters are indicated at12-14, having heads 15-17 coupled at 18-20 to the specimen or load 21.The shakers also include drives 22-24 to which cyclic current ofindicated amplitudes is transmitted as follows:

14(t) 14(SII'I (01+ (#4) IC(I)=IC (Sin (0'+C) (3) The power amplifiers25-27 respectively supply the above drive current.

Phase and amplitude controls for the power amplifiers are indicated at28-30, with outputs at 31-33. These controls receive frequency input tofrom the master oscillator 34, and they may be of the type described atin the above mentioned copending Helmuth application for U.S. LettersPatent, now U.S. Pat. No. 3,477,281, Ser. No. 506,330. In general, theintended function of the controls is to control the phase relationshipsof the cyclic currents, and their relative amplitudes, so as tosubstantially optimize the efficiency of vibration drive of the load 21as described in that prior application. Note in this regard thevibration pick-ups such as accelerometers 35-37 connected to the load atpoints near their respective associated shakers. Each pick-up detectsthe amplitude of the motion of the load at or near its associated shakerdrive point, and the pick-ups produce corresponding signals fed back tothe controls via leads 38-40, to be used for amplitude control of therespective shakers.

As previously mentioned, in the case of resonant, tightly coupledstructures, whenever the vibration pickup at a station (say pickup 35)is influenced by the drive from exciter heads other than the one whichit controls (say the drive from shakers 16 and 17), instability andfailure of control can occur. This is obviated in accordance with theinvention by furnishing a bias or stabilizing signal which is a functionof the vibratory drive applied to the load via at least one of theshakers, and by applying that signal to bias the vibratory drive appliedto the load via another of the shakers, the amplitude and phase of thebias signal being such as to effect reduction of system instability.

In the FIG. 1 example, versions A(t), B(t) and C(t) of the electricaldrive signals to the shakers 12-14 are summed at 40 to provide thesignal EA'(t) B'(t) C(t) at point 41. In this regard, the versions A(t),B'(t) and C(t) may be derived from A(t), B(t) and C(t) by means of gaincontrols or independently adjustable amplifiers indicated at 42-44. Thesum signal at 41 is next inverted at amplifier 45 having a gain controlat 46, to produce the bias signal 2A'(t) B(t) C(t) at point 47. Thelatter signal is then applied to the inputs 48-50 of all the poweramplifiers 25-27 to be superposed upon the respective inputs at 31-33.By virtue of the inversion of the bias signal at 45, and by virtue ofindependent adjustment of one or more of the gain controls 42-44 and 46,the amplitude and phase of the bias signal are found to be such as toeffect reduction of system instability,. Note that any of the gains at42-44 may be independently reduced to zero. Stability may thus beachieved over a range of oscillator inputs (shaker frequencies) byadjustment of gain over that range. Once so determined, gain may beprogrammed as a function of oscillator input, the elements 42-44 and 46also representing such programming.

By way of further explanation of the above phenomenon, reference is madeto FIGS. 20 2d. Considering FIG. 2a, vector 50 represents the amplitudeand phase of the drive input to the load from shaker 12, whereas vector51 represents other input to that portion of the load proximate shaker12, due to cross-coupling of other shakers. The resultant input vectoris seen at 52. In FIG. 2b the amplitude of vector 51 has increased tosuch undesirable extent that vector 50 has the wrong sense, i.e. has anegative component, representing power removed from the load via shaker12. This represents a system unstable condition, which the inventionseeks to remedy.

As implied above, the approach or occurrence of a non-stable conditionmay be suitably indicated, as is conventional. For example, a light incontrol 28 may come on, or a meter in 28 may indicate a change in output31, in the event of such condition. The same applies to controls 29 and30.

FIG. 2c shows the fumishing or generation of an additional vector 50(a)which, added to vector 50(b) produce vector 50 of FIG. 2b. Vector 50(a)represents shaker 12 input due to signal 47 (applied at 48) and vector50(b) represents shaker 12 input due to signal 31 from control 28.Vector 50(a) is typically generated to have angularity such as always toprovide a component in opposition to the desired vibration, ideally 180out of phase with vector 52 (the resultant vibration level). Theamplitude and direction of vector 50(a) are such that vector 50(b) willalways be directed to the right, i.e. will not have a negativecomponent. In the example of FIG. 1 vector 50(a) is represented by thebias signal developed at 47, it being assumed that the drives from thevarious shakers to the load are nearly equal. In FIG. 2d the vectors50(0), 51 and 50(b) add to produce the vector 52, corresponding to astable condition due to forward direction of 50(1)). The operator mayenable the bias signal application by closing of the switch 47a in lead47 when he sees the light referred to above come on.

FIG. 3 shows another form of bias signal development, wherein variousbias signals A(t), B'(r), C(t) are provided. Also, bias signals 2 B'(t)C'(t), E A'(t) C(t) and Z A'(t) B'(t) are provided by the invertingamplifiers 60-62, having inputs 63-65 from the summing apparatus andalso having gain controls 66-68. In this regard, the various biassignals may be selectively and independently fed to the power amplifiersas checked in the following:

In the case of a structure and test set-up characterized assubstantially non-uniform as regards loading imposed on the differentshakers, the bias signal may be furnished by recording the drive signalto one shaker, and the thus recorded signal may be played back forapplication as a stabilizing signal. For example, in FIG. 4, shaker 70may be driven by itself with shakers 71 and 72 being shut-down, theoutput at 95 being controlled at desired phase and amplitude byadjustment of the phase and amplitude control 73, as for example inresponse to feed back from pickup 95 in the manner as previouslydescribed in FIG. 1. A version A(t) of the drive signal from the control73 to the power amplifier 76 may be recorded at 79, as a function offrequency of master oscillator 82, the recorder having oscillator inputat 83. Specifically, the in-phase and quadrature components of A(t) maybe so recorded, the recorder for example being a drum or linear type X-Ydevice with curve follower capability (two channel). In the same mannershaker 71 may be driven with shakers 70 and 72 shut down, and arecording of the stabilizing signal made at 80; and shaker 72 may bedriven with shakers 70 and 71 shut down and a stabilizing signalrecorded at 81. The signal recorded at 79 incorporates the influence ofthe connections of the shakers 71 and 72 to the load upon the detector95 associated with shaker 70; the signal recorded at 80 incorporates theinfluence of the connection of shakers 70 and 72 to the load upon thedetector 96 associated with shaker 71; and the signal recorded at 81incorporates the influence of the connection of the shakers 70 and 71 tothe load upon the detector 97 associated with shaker 72. In this regard,when shaker 70 is driven with shakers 71 and 72 shut down, the shakerdrive to the load corresponds to vector 50 in FIG. 2b and the reactiveforce influence of the shut down shakers on the detector 95 correspondsto vector 51 in FIG. 2b. The signal recorded at 79, properly inverted,corresponds to generation of vector 50(a) seen in FIG. 2c. Finally, theplayback of the recorded signal to the drive 76 corresponds to FIG. 2d,with vectors 50(a), 51 and 50(b) adding to produce vector 52, a stablecondition.

Thereafter playback terminal 79a of recorder 79 may be connected tostabilizing input terminal 76a of amplifier 76; playback terminal a ofrecorder 80 may be connected to stabilizing input terminal 77a ofamplifier 77; and playback terminal 81a of recorder 81 may be connectedto stabilizing input terminal 78a of amplifiers 78. The system is thenoperated with all shakers running and the stabilizing signals applied tothe amplifiers as described, with their levels predetermined asfunctions of oscillator frequency. As before, the various stabilizingsignals may be phase inverted if necessary to subtract from the driveinputs to the amplifiers 76-78. The resultant difierence signals areused to control the shakers.

FIG. 5 shows a system like that of FIG. 1, but incorporatingmodifications in the generation of the bias signals. Elements 25a-34correspond to elements 25-34 in FIG. 1; and the elements above theamplifiers 25a-27a which are not shown are the same as in FIG. 1 withthe exception that the load may not be symmetrical. Versions A(t), B(t)and C(t) of the electrical drive signals to the shakers are fed to eachof three groups of gain controls, i.e. group 42a, 43a and 44a, groups42b, 43b, 44b, and 42c, 43c and 44c. The outputs from those controls arerespectively summed at 40a, 40b and 40c, and inverted at amplifiers 45a,45b and 450 having gain controls 46a, 46b and 460. The outputs at 148,149. and are then respectively applicable, via controls 148a, 149a and150a as inputs 48a, 49a and 50a to the power amplifiers 25a, 26a and270, when approaching instability is detected, as described above. Thispermits separate adjustment of the gains associated with production ofthe bias signals 48a, 49a and 50a to effect reduction of systeminstability under non-symmetrical drive conditions as respects the load.Controls 148a, 149a and 1500 comprise computer memory and programmingcircuitry enabling online or delayed application of the bias signals asdescribed.

FIG. 6 illustrates elements 40f-47f corresponding respectively toelements 40-47 in FIG. 1 (and also to similar elements in FIG. 5);however, in this instance the elements 42f-44 incorporate independentlyadjustable phase as well as gain controls. The summing means 40faccordingly sums the X and Y components of the vectors defined by theoutputs A(!), B'(t) and C'(t), in deriving the bias signal. Here againthe purpose is to provide electrical compensation for unsymmetricalloading of the structure being driven, as where the drive couplings tothe load are not equally spaced apart, or the load geometry varies alongits length, the overall purpose being to achieve stability. Bias signal47f is applied to the memory of computer 147f, and may be applied, underthe control of programming in the computer, to bias one or more of thepower amplifiers as in FIGS. 1 and 5.

lclaim:

1. In the method of stabilizing a system of multiple shakers and drivestherefor operatively connected with a load and wherein the load couplesenergy delivery between the shakers acting to unstabilize the system,said drives including phase and amplitude control, the steps thatinclude a. detecting the amplitude of load motion near the locus offorce application to the load via said one shaker,

b. controlling one phase and amplitude control associated with oneshaker drive in response to said detection for controlling the amplitudeof the output of said one shaker,

c. furnishing a bias signal derived from the output of said one control,said furnishing being carried out with accompanying amplitude controland phase shift of said bias signal, and

d. applying said signal to bias the vibratory drive applied to the loadvia another of said shakers.

2. The method of claim 1 wherein said bias signal is derived as anadditive function of the drives applied to the load via multiple shakersincluding said one shaker.

3. The method of claim 1 wherein said furnishing step includes derivinga summed version of multiple oscillatory drive signals transmitted frommultiple drives to multiple shakers.

4. The method of claim 3 wherein said furnishing step includes invertingthe phase of said summed version thereby to derive said bias signal.

5. In the method of stabilizing a system of multiple shakers and drivestherefor connected with a load and wherein the load couples energydelivery between the shakers acting to unstabilize the system, saiddrives including phase and amplitude control, the steps that include a.detecting the amplitude of load motion near the locus of forceapplication to the load via said one shaker,

b. controlling one phase and amplitude control associated with oneshaker drive in response to said detection for controlling the amplitudeof the output of said one shaker,

c. furnishing and recording a bias signal derived from the output ofsaid one control, and

d. operating all of the shakers and playing back said recorded biassignal to bias the vibratory drive applied to the load via said oneshaker.

6. ln apparatus for stabilizing a system of multiple shakers andvibratory drives therefor operatively connected with a load and whereinthe load couples energy delivery between the shakers acting tounstabilize the system, said drives including phase and amplitudecontrol, the improvement which comprises a. transducer means fordetection of the amplitude of load motion near the locus of forceapplication to the load via said one shaker,

b. feed-back means responsive to said detecting means for controllingone phase and amplitude control associated with one shaker drive forcontrolling the amplitude of the output of one shaker,

c. primary means furnishing a bias signal derived from the output ofsaid one control, said primary means including means to vary theamplitude of the bias signal and to effect a greater than phase shift ofthe bias signal, and

d. secondary means to apply said signal to bias the vibratory driveapplied to the load via another of said shakers.

7. The apparatus of claim 6 wherein said primary means includes acontrol network connected to sum versions of the drives applied to theload via multiple shakers.

8. In apparatus for stabilizing a system of multiple shakers andvibratory drives therefor connected with a load and wherein the loadcouples energy delivery between the shakers acting to unstabilize thesystem, said drives including phase and amplitude control, theimprovement which comprises a. transducer means or detecting theamplitude of load motion near the locus of force application to the loadvia said one shaker,

b. feed-back means responsive to said detecting means for controllingone phase and amplitude control associated with one shaker drive forcontrolling the amplitude of the output of one shaker,

c. primary means furnishing and recording a bias signal derived from theoutput of said one control,

d. secondary means to apply said recorded signal to bias the vibratorydrive applied to the load via said one shaker.

9. The method of claim 3 wherein said furnishing step includes adjustingthe phases of said drive signals prior to said derivation of said summedversion, said bias signal being applied via a computer memory.

10. The method of claim 3 including a. controlling another phase andamplitude control associated with another shaker drive in response tothe detection of the amplitude of load motion near the locus of forceapplication to the load via said other shaker for con trolling theamplitude of the output of said other shaker,

b. furnishing another bias signal by deriving another summed version ofmultiple oscillatory drive signals transmitted from multiple drives tomultiple shakers, and c. applying said other bias signal, via a computermemory,

to bias the vibratory drive applied to the load via said one shaker.

1. In the method of stabilizing a system of multiple shakers and drivestherefor operatively connected with a load and wherein the load couplesenergy delivery between the shakers acting to unstabilize the system,said drives including phase and amplitude control, the steps thatinclude a. detecting the amplitude of load motion near the locus offorce application to the load via said one shaker, b. controlling onephase and amplitude control associated with one shaker drive in responseto said detection for controlling the amplitude of the output of saidone shaker, c. furnishing a bias signal derived from the output of saidone control, said furnishing being carried out with accompanyingamplitude control and phase shift of said bias signal, and d. applyingsaid signal to bias the vibratory drive applied to the load via anotherof said shakers.
 2. The method of claim 1 wherein said bias signal isderived as an additive function of the drives applied to the load viamultiple shakers including said one shaker.
 3. The method of claim 1wherein said furnishing step includes deriving a summed version ofmultiple oscillatory drive signals transmitted from multiple drives tomultiple shakers.
 4. The method of claim 3 wherein said furnishing stepincludes inverting the phase of said summed version thereby to derivesaid bias signal.
 5. In the method of stabilizing a system of multipleshakers and drives therefor connected with a load and wherein the loadcouples energy delivery between the shakers acting to unstabilize thesystem, said drives including phase and amplitude control, the stepsthat include a. detecting the amplitude of load motion near the locus offorce application to the load via said one shaker, b. controlling onephase and amplitude control associated with one shaker drive in responseto said detection for controlling the amplitude of the output of saidone shaker, c. furnishing and recording a bias signal derived from theoutput of said one control, and d. operating all of the shakers andplaying back said recorded bias signal to bias the vibratory driveapplied to the load via said one shaker.
 6. In apparatus for stabilizinga system of multiple shakers and vibratory drives therefor operativelyconnEcted with a load and wherein the load couples energy deliverybetween the shakers acting to unstabilize the system, said drivesincluding phase and amplitude control, the improvement which comprisesa. transducer means for detection of the amplitude of load motion nearthe locus of force application to the load via said one shaker, b.feed-back means responsive to said detecting means for controlling onephase and amplitude control associated with one shaker drive forcontrolling the amplitude of the output of one shaker, c. primary meansfurnishing a bias signal derived from the output of said one control,said primary means including means to vary the amplitude of the biassignal and to effect a greater than 90* phase shift of the bias signal,and d. secondary means to apply said signal to bias the vibratory driveapplied to the load via another of said shakers.
 7. The apparatus ofclaim 6 wherein said primary means includes a control network connectedto sum versions of the drives applied to the load via multiple shakers.8. In apparatus for stabilizing a system of multiple shakers andvibratory drives therefor connected with a load and wherein the loadcouples energy delivery between the shakers acting to unstabilize thesystem, said drives including phase and amplitude control, theimprovement which comprises a. transducer means or detecting theamplitude of load motion near the locus of force application to the loadvia said one shaker, b. feed-back means responsive to said detectingmeans for controlling one phase and amplitude control associated withone shaker drive for controlling the amplitude of the output of oneshaker, c. primary means furnishing and recording a bias signal derivedfrom the output of said one control, d. secondary means to apply saidrecorded signal to bias the vibratory drive applied to the load via saidone shaker.
 9. The method of claim 3 wherein said furnishing stepincludes adjusting the phases of said drive signals prior to saidderivation of said summed version, said bias signal being applied via acomputer memory.
 10. The method of claim 3 including a. controllinganother phase and amplitude control associated with another shaker drivein response to the detection of the amplitude of load motion near thelocus of force application to the load via said other shaker forcontrolling the amplitude of the output of said other shaker, b.furnishing another bias signal by deriving another summed version ofmultiple oscillatory drive signals transmitted from multiple drives tomultiple shakers, and c. applying said other bias signal, via a computermemory, to bias the vibratory drive applied to the load via said oneshaker.